JP3457516B2 - Gallium nitride based compound semiconductor device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a blue-violet semiconductor laser using a gallium nitride compound semiconductor, a high-intensity blue / green light emitting diode, and the like, and in particular, a gallium nitride compound semiconductor with an improved p-side contact portion. Regarding elements .

[0002]

2. Description of the Related Art Conventionally, short wavelength semiconductor lasers are InGa
A 600 nm band light source using an AlP material has improved the characteristics to a level at which reading / writing of a disk is possible, and has already been put to practical use. Therefore, blue semiconductor lasers with shorter wavelengths have been actively developed with the aim of further improving the recording density. This is because the laser beam having a short oscillation wavelength can reduce the focusing size and is effective in increasing the recording density.

GaN, InGaN, GaAlN, InG
Since gallium nitride-based compound semiconductors such as aAlN have a very wide band gap and can be expected to emit light with a short wavelength, they are attracting attention as a material for short-wavelength semiconductor lasers for application to high-density optical disk systems and the like.

For example, in a semiconductor laser using a GaN-based material, pulse oscillation with a wavelength of 380 to 417 nm has been confirmed, but satisfactory characteristics are not obtained, and the threshold voltage in room temperature pulse oscillation is 10 to 40 V. It is a high value and the variation is large.

This is because the crystal growth of the gallium nitride-based compound semiconductor layer is difficult and the device resistance is large. That is, a p-type gallium nitride-based compound semiconductor layer having a high carrier concentration cannot be formed, and the contact resistance of the p-side electrode is high, which causes a large voltage drop, and even a pulse oscillation operation causes deterioration due to heat generation or a metal reaction. caused by.

Further, when a current required for laser oscillation is injected, the p-type gallium nitride compound semiconductor layer is not a crystal of good quality, but has defects having a plurality of fine holes along the growth direction from the lower layer to the upper layer. Therefore, a high current locally flows, so that carriers cannot be uniformly injected into the active layer.
There is also a problem that continuous oscillation does not occur because the element is destroyed instantaneously.

As described above, in order to realize a highly reliable gallium nitride-based blue-violet semiconductor laser which operates at a low threshold current and a low threshold voltage for practical use in optical disks and the like, the active layer must be formed. Although it is important to efficiently and uniformly inject the carrier and suppress the voltage drop at the electrode contact, it is extremely difficult to realize this at present.

[0008]

As described above, in the gallium nitride-based compound semiconductor laser, since the p-side electrode contact resistance is high, a large voltage drop occurs at the electrode contact, which results in a low threshold current and a low operating voltage. It is difficult to realize the device. Further, the contact resistance of the p-side electrode is high, so that the operating voltage is high, and the p-side electrode metal and Ga
It was difficult to continuously oscillate the laser because N reacts when it is energized and deteriorates.

The present invention has been made in consideration of the above circumstances. An object of the present invention is to reduce the contact resistance generated between the p-side electrode and the p-side electrode contact layer. It is an object of the present invention to provide a gallium nitride-based compound semiconductor device which has excellent reliability without causing deterioration at a threshold current and a low operating voltage.

[0010]

[Means for Solving the Problems]

(Structure) In order to solve the above-mentioned subject, the present invention has adopted the following structures. That is, the present invention is a gallium nitride-based compound semiconductor device having a double hetero structure in which an active layer is sandwiched between a pair of clad layers having different conductivity types, which is made of a single crystal gallium nitride-based compound semiconductor. between the p-side contact layer and the p-side electrode, it forms a gallium nitride-based compound semiconductor layer of polycrystalline, the semiconductor
The layer has a thickness of 10 nm or less and a carrier concentration of 1 × 10 ¹⁷
It is characterized by being a p-type having a cm ⁻³ or more .

[0011]

Here, the following are preferred embodiments of the present invention. (1) A single crystal gallium nitride-based compound semiconductor is Ga _x1 I
n _y1 Al _z1 N (x1 + y1 + z1 = 1,0 ≦ x1, y
1, z1 ≦ 1) and the polycrystalline gallium nitride compound semiconductor layer is Ga _x2 In _y2 Al _z2 N (x2 + y2 + z
2 = 1,0 ≦ x2, y2, z2 ≦ 1), and the p-side contact layer has Mg added.

(2) The polycrystalline gallium nitride-based compound semiconductor layer is GaN.

(3) The active layer is a GaN well layer and an AlGaN
It has a multiple quantum well structure in which barrier layers are alternately stacked. (Function) The contact resistance generated in the p-side electrode and the p-side contact layer (for example, GaN) is due to the barrier existing at the interface and having a height close to 2 eV. By reducing the height of the barrier, the current during application easily flows, and the contact resistance can be reduced. The height of this barrier depends on the material of the p-side electrode used, and the higher the work function of the metal, the smaller the barrier height. However, since the band gap of the gallium nitride based semiconductor itself is large, even if a metal material is selected. The effect is small.

Further, the band of a gallium nitride compound semiconductor such as p-type GaN is largely bent on the surface in the direction of increasing the barrier height, and the extent thereof depends on the surface condition, that is, impurities and crystallinity existing on the surface. It turned out to be dependent.

In the present invention, a gallium nitride-based compound semiconductor (Ga _x1 In _y1 Al _z1 N: x1 + y1 + z1: 1,0 ≦
x1, y1, z1 ≦ 1), and in a gallium nitride-based compound semiconductor device in which the active layer is sandwiched by semiconductor layers having different conductivity types, the barrier height apparently generated between the p-side contact layer and the p-side electrode is apparent. In order to reduce it, a level is deliberately provided at this interface to allow a current to pass.

A conceptual diagram is shown in FIG. (A) is a conventional structure showing a junction between p-GaN and an electrode metal.
There is a Schottky barrier near eV. here,
By forming an interface level as shown in (b) or changing the surface state, the bending of the band near the surface is also reduced, and a current easily flows through the interface level, resulting in a contact resistance. Get smaller.

Specifically, on the surface of the p-side contact layer,
Polycrystalline Ga _x2 In _y2 Al _z2 N (x2 + y2 + z2 = 1,
By forming 0 ≦ x2, y2, z2 ≦ 1), the bending of the band is reduced, the interface state is further formed, and the contact resistance with the p-side electrode is reduced.

Here, when the polycrystalline layer is inserted between the p-side electrode material and the p-side contact layer, the leakage current increases and the apparent resistance decreases when the polycrystalline layer has a thickness of 10 nm or more. However, it is impossible to realize a gallium nitride-based compound semiconductor laser that does not deteriorate at a low threshold current and a low operating voltage and has excellent reliability. Therefore, it is desirable that the thickness of the polycrystalline Ga _x2 In _y2 Al _z2 N layer is 10 nm or less.

The polycrystalline Ga _x2 In _y2 Al _z2 N layer is
It is desirable that the p-type semiconductor is 1 × 10 ¹⁷ / cm ³ or more by adding impurities. The reason why the impurity concentration of polycrystalline GaInAlN is preferably 1 × 10 ¹⁷ / cm ³ or more is that if it is less than that, the band bending may move in the direction of a large barrier height, and the resistance of the inserted polycrystalline itself. It also becomes large, which is not preferable for reducing the contact resistance.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a sectional view showing the schematic arrangement of a blue semiconductor laser according to the first embodiment of the present invention.

On the sapphire substrate 1, a GaN buffer layer 2, an n-type GaN contact layer 3 (Si-doped; 5 × 10 5).
¹⁸ / cm ³ , thickness 4 μm, n-type Al _0.2 Ga _0.8 N cladding layer (Si-doped; 5 × 10 ¹⁷ / cm ³ , thickness 0.
3 μm) 4, GaN waveguide layer (undoped, thickness 0.1 μm
m) 5, MQW active layer 6, GaN waveguide layer (undoped or Mg-doped, thickness 0.1 μm) 7, p-type Al _0.2 G
a _0.8 N cladding layer (Mg-doped; 5 × 10 ¹⁷ / cm
³ , thickness 0.3 μm) 8, p-type GaN contact layer 3
(Mg-doped; 1 × 10 ¹⁸ / cm ³ , thickness 1 μm) 9 are formed in the above order. The MQW active layer 6 is G
It is formed by alternately stacking aN well layers and AlGaN barrier layers.

A p-type polycrystalline GaN layer 10 having a thickness of 5 nm is formed on a part of the p-type GaN contact layer 9 having the above-mentioned multilayer structure, and a Pt film 11 having a thickness of 10 nm and an Au electrode pad 12 having a thickness of 1 μm are formed thereon. Are sequentially laminated, whereby a p-side electrode is formed.

Further, a part of the outermost surface of the p-type GaN contact layer 9 is removed by a dry etching method to a depth reaching the n-type GaN contact layer 3, and the exposed n-type electrode Ti / Au13 is formed.

Next, the manufacturing method and operation of such a blue semiconductor laser will be described. In FIG. 1, each layer from the GaN buffer layer 2 to the p-type GaN contact layer 9 on the sapphire substrate 1 is formed by one MOCVD growth.

Next, on the surface of the p-type GaN contact layer 9, polycrystalline GaN having a thickness of 5 nm is formed in a region having a width of 10 μm by a vapor deposition method. Subsequently, when heat treatment is performed in a nitrogen atmosphere at 300 ° C., excess Mg in the p-type GaN contact layer 9 diffuses into the polycrystalline GaN portion, and p-type polycrystalline Ga is formed.
The N layer 10 is formed. Further, the heat treatment at 300 ° C. makes the p-type polycrystalline GaN layer 10 into a polycrystal having multiple orientations and uniform grain agglomerates. On top of that, a 10 nm Pt film 11 is further formed.
And 1 μm Au pad 12 are sequentially laminated to form a p-side electrode.

Next, to form the n-side electrode 13, a mesa shape including the p-side electrode is formed, and the n-type G that appears under the mesa is formed.
An n-side electrode 13 made of Ti / Au on the aN contact layer 3
To form. Here, the p-side electrode may be formed after forming the n-side electrode 13. After this, the sapphire substrate 1 is 50μ
It is mirror-polished to m and is cleaved in a direction perpendicular to the longitudinal direction of the p-side electrode to form a 1 mm-long laser chip.

The blue semiconductor laser thus formed is
Continuous oscillation at room temperature was performed with a threshold current of 80 mA. The oscillation wavelength is 420 nm, the operating voltage is 7 V, and the temperature is 50 ° C and 3
The device life at 0 mW drive was 5000 hours.

In the semiconductor laser according to the embodiment of the present invention, since the p-type polycrystalline GaN layer 10 is formed between the p-type GaN contact layer 9 and the p-side electrodes 11 and 12, the above-mentioned structure is formed in the contact portion. An interface level as shown in FIG. 2 (b) is formed, and a current easily flows through this interface level to reduce the contact resistance. Furthermore, p-type polycrystalline G
Since the aN layer 10 is as thin as 5 nm, there is no inconvenience such as an increase in leak current. For this reason, it is possible to realize a gallium nitride-based compound semiconductor laser which has a low threshold current and a low operating voltage and does not deteriorate and has excellent reliability.

The current-voltage characteristics of the semiconductor laser according to this embodiment are shown in FIG. A curve 31 in the figure is the characteristic of the semiconductor laser according to the present embodiment. Curve 32 in the figure
[FIG. 4] shows, as a comparative example, the current-voltage characteristics of the semiconductor laser not including the polycrystalline GaN layer 10 in the above embodiment. As is clear from this characteristic curve, the characteristic of the curve 32 is not a perfect diode characteristic,
The voltage rise was also very high at about 15 V, and although light emission was confirmed, it deteriorated within a few minutes.

Although the structure is the same as that of the present embodiment, as a different manufacturing method, as a p-type GaN contact layer 9 is grown by MOCVD, the temperature is lowered in the MOCVD apparatus as it is. Then, the polycrystalline GaN layer 10 having a thickness of 5 nm is formed by continuously performing low temperature growth. At this time, p-type GaN is formed on the polycrystalline GaN layer 10.
Similar to the contact layer 9, Mg is added by DMG. Also in this case, it was confirmed that the same effect as that obtained when the polycrystalline GaN layer 10 was formed by vapor deposition was obtained.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
2 is a cross-sectional view showing a schematic configuration of a blue semiconductor laser according to the embodiment of FIG. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

The semiconductor laser of the present embodiment is intended to further reduce the contact resistance as compared with the first embodiment. Specifically, as shown in FIG. 4, the p-type GaN contact layer 9 is formed. P-type In as a contact layer
_{The 0.1} Ga _0.9 N layer 41 is inserted, and the polycrystalline GaN layer 10
Instead of this, a polycrystalline p-type In _0.1 Ga _0.9 N layer 42 is formed. The other configuration is exactly the same as that of the first embodiment.

In the case of the laser of this embodiment, p-type InGaN is used.
The layer has a narrower band gap than the p-type GaN single crystal 9 and the p-type GaN polycrystal 10 both in the case of the single crystal 41 and in the case of the polycrystal 42, so that the Schottky barrier becomes low and the electrode contact resistance is the first. A reduction of 20% was achieved compared to the embodiment.

As described above, according to this embodiment, since the p-type polycrystalline InGaN having a narrow band gap is similarly inserted between the p-type InGaN contact layer having a narrow band gap and the Pt electrode, the first In addition to the effects of the embodiment,
The contact resistance with the p-side electrode can be further reduced, and thus the operating voltage can be further reduced. In this case, the p-type InGaN layer 41 and p
The In composition in the type polycrystalline InGaN layer 42 need not be the same.

(Other Embodiments) The present invention is not limited to the above embodiments. The composition and film thickness of the semiconductor layer described in the embodiments are merely examples, and can be appropriately changed according to the specifications. Further, the semiconductor layers may have the opposite conductivity type.

For example, the polycrystalline gallium nitride-based compound semiconductor layer to be inserted between the p-side contact layer and the p-side electrode is not limited to GaN or InGaN, but the general formula G
It may be any as long as it is defined by a _x2 In _y2 Al _z2 N (x2 + y2 + z2 = 1,0 ≦ x2, y2, z2 ≦ 1). The film thickness is 10n
The carrier concentration is 1 × 10 ¹⁷ cm ^-3.
The above is sufficient.

Although the semiconductor laser has been described as an example in the embodiment, the present invention can be applied to a light emitting element such as an LED. Further, in addition to the light emitting element, it can be applied to a light receiving element using a gallium nitride compound semiconductor, an electronic device such as a transistor, and the like. In addition, various modifications can be made without departing from the scope of the present invention.

[0039]

As described above in detail, according to the present invention, p
A thickness of 10 is desirable between the side electrode and the p-side contact layer.
by inserting a p-type gallium nitride-based compound semiconductor layer having a carrier concentration of 1 nm or less and a carrier concentration of 1 × 10 ¹⁷ cm ⁻³ or more,
A low resistance p-type contact can be stably realized.
As a result, it is possible to realize a gallium nitride-based compound semiconductor device having excellent reliability without causing deterioration at a low threshold current and a low operating voltage.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a first embodiment.

FIG. 2 is a diagram showing a concept of a band structure in an electrode and a contact layer of the present invention in comparison with a conventional example.

FIG. 3 is a diagram showing current-voltage characteristics of the semiconductor laser in FIG. 1 together with a comparative example.

FIG. 4 is a sectional view showing a schematic configuration of a blue semiconductor laser according to a second embodiment.

[Explanation of symbols]

1 ... sapphire substrate 2 ... GaN buffer layer 3 ... n-GaN contact layer _{4 ... n-Al 0.2 Ga 0.8} N cladding layer 5 ... n-GaN waveguide layer 6 ... active layer 7 ... p-GaN waveguide layer 8 ... p -Al _0.2 Ga _0.8 N cladding layer 9 ... p-GaN contact layer 10 ... p-GaN polycrystal layer 11 ... Pt film 12 ... Au electrode pad 13 ... n-side electrode 41 ... p-InGaN contact layer 42 ... p-InGaN poly Crystal layer

Continuation of front page (56) Reference JP-A-9-186364 (JP, A) JP-A-9-312416 (JP, A) JP-A-6-252514 (JP, A) JP-A-6-338632 (JP , A) JP 8-330629 (JP, A) JP 6-252163 (JP, A) JP 2-54591 (JP, A) JP 11-4039 (JP, A) JP 10-242587 (JP, A) JP 9-191160 (JP, A) JP 10-65216 (JP, A) JP 9-289351 (JP, A) JP 9-129984 (JP, A) JP 10-214998 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 33/00 H01S 5/00-5/50 H01L 21/28-21/288 H01L 29/40-29/64

Claims (2)

(57) [Claims] 1. A gallium nitride-based compound semiconductor device having a double hetero structure, which is made of a single-crystal gallium nitride-based compound semiconductor and has an active layer sandwiched by clad layers having different conductivity types. A polycrystalline gallium nitride-based compound semiconductor layer is formed between the contact layer and the p-side electrode , and the semiconductor layer has a thickness of 10 nm or less.
A gallium nitride-based compound semiconductor device having a p-type concentration of 1 × 10 ¹⁷ cm ⁻³ or more . 2. The single crystal gallium nitride-based compound semiconductor is Ga _x1 In _y1 Al _z1 N (x1 + y1 + z1 = 1,0).
≦ x1, y1, z1 ≦ 1), and the polycrystalline gallium nitride-based compound semiconductor layer is Ga _x2 In _y2 Al _z2 N
(X2 + y2 + z2 = 1,0 ≦ x2, y2, z2 ≦ 1)
2. The gallium nitride-based compound semiconductor device according to claim 1, wherein the p-side contact layer is formed by adding Mg.